My Iphone rings twice and then goes to voice mail, how do I stop this from happening?

I've had my IPhone 4 for nearly 2 years and in that time I have experienced a lot of minor problems - some are still happening! But the biggest one is regarding when the phone rings as this is now causing my a problem when people are try to contact me instantly! It will only ring 2 and 1/2 times then switches to voice may. I cannot find anywhere on the fone to stop this from happening. How do I change to a longer ring time?

Also, how do I use a song as a ringtone? This is some thing else I would like to know?

Many Thanks

ART